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What is a laminar airfoil and what are their pros and cons?
I am wondering what is a laminar airfoil and what are their advantages and disadvantages if compared to other designs.

You want the complete or concise version

The fist decades of aviation used empirically determined airfoil shapes which usually had most camber near the nose. Such airfoils tend to have pressure peaks near the nose, followed by a long and shallow pressure recovery. The distinction between laminar and turbulent boundary layers was not known, so airfoil shapes did not factor in boundary layer transition.
In the late 1930s, when the first wind tunnels with reduced turbulence became operational, new insights were gained in how to further reduce friction drag. From this NACA history page:

The low drag coefficients achieved by internally braced monoplanes equipped with retractable landing gears suggested that any further large reductions in drag could only be achieved through the maintenance of extensive laminar flow over the surfaces of the aircraft. The boundary-layer flow of contemporary aircraft was essentially all turbulent; and since the skin friction coefficients for turbulent flow are much higher than those for laminar flow, the achievement of laminar flow on the surface of the aircraft would be expected to yield large reductions in drag. For example, the skin friction coefficient on a flat plate is reduced by a factor of almost 2 as the point of transition from laminar to turbulent flow is moved from the leading edge to the 50-percent-chord location.

Better understanding in the causes of boundary layer transition and improved analytical methods allowed researchers to tailor airfoils which showed a delayed laminar to turbulent transition over a small angle of attack (AoA) region, so a reduced drag coefficient in that range could be achieved. This is done by shaping the airfoil surface for a desired lengthwise pressure distribution which shows on the upper side a short, steep pressure drop at the nose, followed by an extended region of shallow pressure drop to about mid chord on both sides. Such a pressure distribution helps to stabilize the laminar boundary layer, thus reducing friction drag, and results in a backward shift of the point of maximum thickness.
Outside of this AoA region, the suction peak on the upper side (at high AoA) rsp. the lower side (at low AoA) forces an early transition on that side which shows up in the drag polar as a sudden jump to higher drag coefficients. The region of reduced drag coefficients between both points is called the laminar bucket of the airfoil polar.

This plot (made with XFOIL 5.4; own work) shows how the laminar bucket is shifted up and down the c$_L$ range by different flap deflections without affecting drag much except for the -20° setting - here, the suction peak on the lower side in combination with the strong flap camber is too much for the boundary layer to remain attached. But this setting is really useable for inverted flight only.
Laminar airfoil pros:

Low drag at an intermediate AoA range. That is the reason why all modern gliders and most modern GA designs use laminar airfoils.

Laminar airfoil cons:

They work as intended only at Reynolds numbers between 1 Million and 5 Million - above that the transition happens very early, regardless of pressure distribution. Since the stability of the laminar boundary layer decreases with higher Reynolds numbers, a  higher Reynolds number narrows the laminar drag bucket and reduces the advantage of laminar over turbulent flow.

Susceptibilty to bugs and rain: Early laminar aifoils used the steepest possible pressure recovery after transition, and an earlier transition triggered by surface imperfections like bugs or raindrops would lead to much earlier flow separation.

Wing sweep will lead to early transition anyway, so large sweep angles will render laminar airfoils ineffective.

High demands on surface perfection: A wavy surface or a gap in the metal sheets for covering the wing will also cause an early laminar-to-turbulent transition, so again a laminar airfoil will be rendered ineffective when combined with shoddy workmanship.

At Reynolds numbers below 1 Million the stability of the laminar boundary layer becomes too high and transition needs to be triggered deliberately in order to get the best performance and avoid large-scale laminar separation. Simply put, almost any airfoil is a laminar airfoil at low Reynolds numbers but performs better with a turbulent boundary layer in the pressure recovery region.

What happens at Re < 1E6? It's easy to see they don't perform, but why?

@Zeus At lower Re you need to trigger transition deliberately in order to get the best performance and avoid large-scale laminar separation.

@ymb1: What is defined as zero flap deflection is a bit up to the airfoil design: Give it lots of rear loading and not much more positive deflection is realistically sensible. And yes, negative is trailing edge up. With a lot of rear loading the -20° is still useful while the +20° will simply show separated flow for most of the AoA range on the upper side. Practical example? Not many; maybe a glider with aerobatic capability.

Not sure if I missed something, but since bugs/rain would make the transition to turbulence happen sooner, why would separation also happen sooner? I thought turbulence helped flow stay attached, not separate? Thanks.

@Wyatt Turbulence causes a much thicker boundary layer and thicker boundary layers are less capable to overcome a steep pressure rise. If transition happens at, say, 45%, the laminar boundary layer until then is thinner, so the now turbulent boundary layer also starts thinner and it takes a steeper pressure rise before it separates, compared to a boundary layer which is turbulent over the whole chord.

Ah okay that makes sense. I'm assuming the reason the thicker BL (more turbulent) would separate first because it's thicker and therefore has a lower speed closest to the wing, and therefore separates easiest?

@Wyatt The thicker boundary layer makes it harder for the outside flow to push the air closest to the wall along - this flow is more isolated and its speed will bleed off more quickly once pressure rises. Once speed drops to zero at the wall, the flow separates.

Ah I see, makes sense. Thanks again!


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Why, with a constant speed propeller, does moving the propeller control forward increase drag?
Could someone explain why, let's say in a constant speed propeller, with  moving propeller control forward the drag increases? It is noticeable when we push the blue knobs forward in flight that the airplane has an additional portion of drag.

If you move the propeller control you increase thrust, therefore increasing speed and therefore increasing drag. Maybe this is an oversimplification to your question or I didn't understand it correctly.

How do you know there's an "increased portion of drag" on the airplane? I don't know about any drag-o-meter....

What I mean is that it is feeled by the crew that with moving propeller controls forward aircraft slows down a bit

@Sanchises when you are landing the plane and you switch from cruise propeller angle to full-power angle, the instant deceleration which accompanies the engine's increase RPM is very noticeable.

You really need to clarify what you're asking.  Obviously moving the propellor control changes the pitch of the blades, and thus (if no engine power is supplied) the drag they create.  Why the designers chose "forward" as the direction of increased drag is another question.

@stevederekson555:  I don't think the OP is asking about increased airframe drag at higher airspeeds, but about the decelerating effect of flattening the prop pitch.  Unless I misunderstood...

@jamesqf: Pushing prop control and power levers both full forward when max power is needed is a pretty obvious design choice. When or why might you want it the other way?

So, you're really asking about what causes the deceleration of the airframe. It's quite an assumption to think this is due to an increase in drag and not due to a decrease in thrust...

@Sanchises:  If the question was about decelerating when you pull the power lever back it would be safe to assume the answer would have to do with decreasing thrust.  Since it is about pushing the prop lever forward I think it is safe to assume the answer has to do with increasing drag.  (see my answer below and comment if you disagree)

@Sanchises that's an interesting comment. Anecdotally, at idle the engine is already producing no thrust and that is when you feel the sharp deceleration. Changing the prop pitch in preparation for landing results in a much sharper than you feel when simply rapidly pulling the throttle back to idle.

@Michael Hall: Perhaps the obviousness is the resut of experience.  That is, the obviousness is because that's what you've worked with.  To me, it seems a bit counter-intuitive.

@jamesqf, I agree it isn't totally obvious, and it is counterintuitive to push the prop control forward to slow down.  I guess I mean that it might be more accurate to say that you don't normally push the prop up "in order to slow down", instead you push it up "because you are slowing down" and want the better power response when you need it.  (i.e. go around)  Actually using it to decelerate is kind of a technique, or byproduct.  Again, it's a bit like downshifting a car:  You know you will need 1st gear at the light, do you coast up and then shift, or shift early and benefit from the decel?

@Michael Hall: Downshifting (or upshifting) is a good example of what I meant by convention.  That is, why should shifting from 1st to 2nd require pulling the shift lever back, from 2nd to 3rd moving it forward and right, &c?  It could easily be set up to work in the opposite direction, or entirely differently, as with motorcycle shifters.

@jamesqf, car comparisons are never perfect, but the point of the conventional power and prop lever function/design, (in place since variable pitch props became a thing...) is that when you want max performance you shove both levers forward.  In an emergency stall recovery or missed approach scenario you want it this way.  Because this IS intuitive.  You wouldn't want to have to remember to push one forward and pull the other back, there would be way too much risk of confusion and error...

@Michael Hall: I agree you wouldn't want them moving in different directions, but to me, pushing them forward to increase power is less "intuitive" than pushing them forward to shut things off.

@jamesqf, well I really can't argue if that is your perspective, but "push forward to go forward" is a deeply ingrained convention for operating many different modes of transportation.

@Michael Hall: Maybe I've spent too much time riding motorcycles :-)

The prop control sets the engine RPM. Moving it forward allows the governor to flatten the blades, which results in a lower angle of attack. In fact, in certain circumstances the AoA goes so low that it actually becomes negative, meaning that the airflow is moving faster through the propeller than the propeller is moving forward. This is like running around with a pinwheel.

In our case, propellers will generally always have a difference between the effective pitch and the geometric pitch[*]. When the effective pitch is shorter then the prop is making positive thrust and when it is longer the prop is making negative thrust.
We can immediately infer that the sharp deceleration felt when the prop control is sped up is due to the effective pitch being longer than the geometric pitch. This is how we know the AoA is negative.
Effects of Negative AoA

@MichaelHall explained it well in his answer, so I won't go into it much here. In short, the negative AoA means that instead of the high pressure area being the underside of the blade, it becomes the topside. This inverts the $F$ vector direction, and means that the propeller is no longer absorbing torque from the shaft, but is instead transmitting torque to the shaft.
Where is the energy going?
The prop is spinning faster, true, but why is this slowing the plane down? After all, $E = F d$, and so if there's a deceleration Force, there must be somewhere that Energy is flowing.
The answer is two-fold. A small amount of energy goes to parasitic drag losses in the propeller, but the vast bulk of it goes to the engine pumping and friction losses.

Simplifying things somewhat[**], for a classic gasoline ICE the energy lost per ideal cylinder cycle is $E = p v$. If we have a bog-standard $360\rm{in}^3$ engine, such as the venerable Lycoming O-360, then, converting into standard units, and assuming a 75% vacuum behind the closed throttle plate, we have E = 101300[Pa] * 0.75 * 5.90[L]/1e3[L/m^3] = 448J per cycle. Since the Lycoming is a four stroke, a piston cycle takes two revolutions, and thusso at 2700RPM-- a typical max speed--, we have 1350 cycles/minute, or 22.5cycles/sec.
22.5 * 448 = 10kW, or approximately 13hp.
Considering that the 0-360 only makes 180hp, an engine braking factor of approximately 10% of peak thrust is certainly going to make itself felt.
While we haven't taken into account internal friction nor parasitic drag, nor deviations from an ideal Carnot engine, we can still see that the primary power sink is the engine pumping air against a closed throttle plate.
This engine-braking effect is very familiar to motorcycle riders and stick-shift drivers, and the same principles apply. Interestingly, this does not apply to diesels, as they do not have a throttle plate. They have far lower engine braking forces and diesel trucks have to go to some length to create engine braking force, which is a useful backup to traditional friction brakes.
[*] Read about the fascinating and ingenious method to achieve true 0-thrust, required in order to empirically test a powered plane's glide ratio: https://engineering.purdue.edu/~andrisan/Courses/AAE490A_S2010/Buffer/AIAA-46372-872.pdf

[**] We assume no friction losses and minimal heat transfer.

Sorry, but I could give you only one upvote. Great answer!

Agreed, big improvement over version 1.0, and I like it better than my own...  (although I think mine has some merit as a math-free alternative!)

@MichaelHall Team effort! My answer steers readers to your answer for an explanation of why the AoA goes negative.

Let’s take the engine out of the equation to simplify things.  In fact, let’s take the prop out too, and consider a very elementary comparison – the old “hand out the window” example...
Put your hand out the window when you are driving and feel the drag it creates.  Now rotate your hand.  Can you feel how there is little drag when your hand forms a “knife edge” versus when your palm is flat, and perpendicular to the airflow?  (I know you know this and I'm not trying to insult anyone's intelligence, but I find it useful sometimes to pull way back to the basics to make a point...)
This is the same principle at work when you vary the pitch of the prop.  When you move the prop control aft towards course pitch you are decreasing the surface area of the prop that is facing the airflow, decreasing drag.  (remember, we are considering a free spinning prop here, in a gliding descent, because I took the engine power out of the example for now…)  With a full feathering prop it is possible to “knife edge” the prop blades, creating very little drag.
Now consider the opposite, that we move the prop control full forward.  This has the effect of flattening the blades to a fine pitch.  If we go far enough they form a slow rotating disc, with a lot of surface area exposed to the oncoming wind, creating lots of parasite drag.  (just like the palm of your hand.)
Let’s add the engine back in - During take off and climb we select a fine pitch, but it isn’t to create drag, it is to get the most power at high RPM and slower airspeeds when we need it.  Just like shifting your car or bike to a lower gear when going up hill.  Normally in cruise or descent we pull the prop lever back to select a coarser pitch for high speed lower RPM efficiency, just like you might use overdrive on the freeway.
When the prop is under load from the engine it is producing power.  The blades have a positive angle of attack, and they are taking “bites of air” and accelerating them rearward to create thrust.  However, in a descent with the power at idle, the reverse might be true.  The angle of attack can be negative as the props extract energy from the airstream to rotate instead of adding energy.  This contributes to the decelerative effect of the drag.
Basically by varying the blade angle and engine power setting we can use the prop to either pull ourselves forward, or to resist forward motion.
So, when might we use the prop to create more drag and resist forward motion?  Well, on a descent at flight idle when trying to slow down to landing gear and flap extension speed.  Without speed brakes it can sometimes be a challenge to slow down, but if we move the props to a fine pitch they flatten out and become a very efficient drag device.
I know, it feels counterintuitive to shove the props up to slow down, but trust me it works quite well.  It's similar to the idea of downshifting your car on a steep decline instead of riding your brakes.  (and it sounds from your question like you have experienced this effect firsthand...)

I have my doubts that this explanation is correct. The angle of attack for a freely spinning propeller blade is going to be very well low. True, additional propeller parasitic drag is created by virtue of the prop spinning faster, but I suspect these losses are negligible in face of the pumping losses from engine braking at several thousand RPM.

@Kenn Sebesta, I am open to suggestions to improve this answer, but a prop under power will have a positive AOA as it "bites" the air and pulls the aircraft forward, while a free spinning blade will have a negative AOA from the force of the airflow making it turn.  Excellent point about the fact you can't ignore the effect of the engine turning because the prop really isn't spinning free, but my gut tells me that the primary contribution to deceleration is the increased drag from the flat pitch.

And might the answer be different between a turboprop, (the basis of my answer) and a piston engine? (the basis of your answer) Because a turboprop is closer to a free spinning blade, while a piston engine has the resistance of compression, which puts it much closer to the example of downshifting a car or motorcycle.

We agree that the AoA is negative, but the disagreement is whether a free-spinning prop has a negative AoA to the point of producing much thrust. My reasoning is that it will only be negative to the extent that energy is required to turn the prop until the prop no longer slips, i.e. the advance ratio goes to 1. The energy required for this is a function of parasitic drag and shaft torque. The parasitic drag is likely very small so the majority of the energy is spent spinning the shaft.

I have no experience in a turboprop, but it sounds like you have an equivalently sharp deceleration event. Are the turbine blades and prop blades set up such that they have a very similar slip ratio? If not, then it might be the case that the turbine blades are also pumping a fair amount of air. It's an interesting question!

I suspect it's completely resolvable through first principles. Maybe @PeterKämpf could weigh in?

@Kenn Sebesta: I respect Peter's IQ and scholarly acumen, but I am a simple unfrozen caveman pilot and prefer common language explanations based on relatable observation over complex equations. ;)  However, your remark about "pumping air" leads me to believe there might be a better answer than simple drag. That the prop is extracting energy from the airflow rather than adding energy to create thrust. I'm just not sure where to go with that idea....

@KennSebesta I just upvoted Michael's answer. And the explanation of the prop extracting energy from the airflow is spot on. Basically, the prop blades have now a negative AoA and drive the engine (compressing air all the time). Just like a car in small gear going downhill.

Thanks for the feedback from both of you!  I added a paragraph that amplifies these points somewhat, please let me know if you have any suggestions on wording.

I think I see your point now, and I did not make this clear in my answer because I took it for granted that the reader would understand why the prop was spinning faster. Thus what was important to understand for me was where the energy was going (because E = $F*d$, so without $E$ we have no $F$). This answer still doesn't really address the fact that the engine losses are where the energy is going, and I think that's a crucial point to understand.

@PeterKämpf, Michael Hall, I revised my answer and worked out the math. The engine braking for an O-360 is an astounding 13hp! The prop spinning faster isn't what's causing the deceleration on its own, it's the negative AoA (which M. Hall explains well) which is being driven by the piston engine braking. Without the braking, the AoA would likely be scarcely negative. I'm not sure the magnitude is the same for a turbine, though, since turbines and diesels share the common trait of not having a throttle body.

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What is the difference between a service manual and a maintenance manual?
What is the difference between service manual and maintenance manual? Because sometimes I found one, sometimes I found another, the content is similar but not the same. What are the differences and purpose of each document?

I don't see the definition in the FAA site.  But based upon experience in military aviation we look at them as synonymous terms. The terms differ at the frequency and level of care provided by maintenance technicians.
https://www.faa.gov/guidance/
Maintenance manuals are aligned with the term Operations & Maintenance manual, which is for daily operations maintenance and corrective actions.
Service manuals are for longer interval maintenance practices when the aircraft is in the hangar undergoing longer maintenance projects.
Ex:  engine flush vs engine disassembly

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Why is the alternator connected only to one electrical bus in the C152?
I am a noob in electrics so pardon my ignorance. What I see from an electrical system of C152 (pasted below): there are two busses, connected to each other, but alternator is fed only to the first bus. Does it mean that each component connected to the second bus takes energy from the battery and due to that battery is constatly discharging and alternator must additionally load the battery? If that is true (correct me if I'm wrong please) why don't connect alternator to both busses?

There's a direct connection from the primary bus to the other bus, so they are essentially one.

So the current from the alternator goes from the one bus to another?

Yes, if you have a wire connecting the two current will flow from one to the other. I'm not sure why they did it that way rather than having a single bus, that's a different question.

Usually, there is a way to disconnect buses from one another, either automatically or manually. The diagram does not show anything on the connecting wire though...

It could just be simplified, if you can't do anything about it in the cockpit you don't really need to know about it sometimes.

@GdD, isn't the connection between the buses the “avionics” switch? It looks like it from the list of connected items.

Note that while the battery is drawn beside the secondary bus, all the connecting lines from it lead to the primary one.

I've never seen a 152 with an avionics master switch @JanHudec.

@GdD, hm, it is in 172 and you are told to have it off when starting the engine, so I supposed a 152 has it too, but seems like I was wrong.

On a 152 you turn all the avionics off at the device level before shutdown @JanHudec

Consider them to be a single bus.  There are two separate buses joined by a "bus tie" wire, either for packaging purposes if the two are together on the firewall (to shorten the linear space used) or because the second bus is mounted somewhere else (like on the back of the firewall in the cockpit) for wiring efficiency.  Although not shown, there could be a fuse link in the bus tie wire between the two, to protect the primary bus and it's higher-importance services from short circuits in services running off the second bus (gotta protect that cigarette lighter!).
With everything connected together, both buses are energized by the alternator if it's running, and the battery if the alternator is off line.  Whichever source's voltage is higher will be supplying the juice.  The alternator runs somewhere around 28v, and the battery is somewhere around 24 ish, so when the engine is running you'll measure 28v +/- at any point of the bus, and the battery just sits there getting and staying fully charged, and when off, it drops to the battery voltage of around 24 +/-, and will decay as the battery's charge is consumed (things will start to stop working as the battery drops below around 20-22 volts).
The hydraulic analogy is limited, but is useful to mentally picture things.  You have two hydraulic manifolds interconnected, and one of the manifolds is connected to a 28 psi hydraulic pump, as well as a 24 psi accumulator.  When the pump is running, the manifolds will be at 28 psi, and the excess pressure being made by the pump above the accumulator's 24 psi operating pressure ensures it stays fully charged.  If the pump is turned off, the system is pressurized by the 24 psi accumulator and the system pressure at any point on the manifolds drops to 24 psi and decays from that point as the  accumulator is depleted.

There's a direct connection from the primary bus to the other bus, so they are essentially one. If you have a wire connecting the two, the current will flow from one to the other.


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What are the EASA myopia limits for civil flight certification, class 2?
I want to start procedures to get a flight medical certification (class 2).
I am not sure about myopia (short-sight) limits.
In some resources I read that there are no limits, and you can wear glasses. In other resources I read that they are very strict.
Could someone who knows, please clarify this confusing topic?
Thank you very much.
EDIT:
This is about Greece

@Bianfable Done! I edited the question. Thank you!

The EASA class two requirements are (p.99):

(2) For a class 2 medical certificate:
(i) Distant visual acuity, with or without correction, shall be 6/12
(0,5) or better in each eye separately and visual acuity with both
eyes shall be 6/9 (0,7) or better.
(ii) Notwithstanding point (b)(2)(i), applicants with substandard
vision in one eye or monocularity may be assessed as fit, in
consultation with the medical assessor of the licensing authority and
subject to a satisfactory ophthalmological evaluation.

Note that it says "with or without correction", which means that you can wear glasses or contact lenses.

Thank you, but i don't understand what is the point to a myopia limit if you can wear glasses. You can have 10 myopia, and with glasses, you drop to zero. So what exactly does this mean?

@user1584421 I don’t know about EASA, but the US has more or less the same wording. Here it has two effects: if someone can’t meet the myopia limit even with glasses then they can’t get that class of medical. Second, if they need glasses to meet the limit then there will be a restriction on their medical that requires them to wear glasses while flying.

EASA Part-MED regulations are non black and white. There is some margin up to the discretion of the AME and Optometrist. If you are outside of the limits, you may be asked to undergo further examinations.
Corrections (Spectacles/contact lenses) are allowed and mandatory depending on your non-corrected visual acuity.


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Where is the pitot static drain located on a Piper PA-28?
Some PA-28 airplanes have pitot static drain where the pilot pushes 2 buttons to open the system.
But where does the water drip out from the airplane? Is there a hole underneath the airplane? Where?
I mean when you push the buttons where should you look for water coming out?

Are you thinking of the alternate static system inside? The static port is usually next to the pitot tube, with the drain underneath on Warriors. I've never seen any buttons in the cockpit - can you provide for info

This presentation gives a nice overview of various systems on the Piper Cherokee series. The buttons you describe are shown here:

(slide 9)
The full pitot-static tube including the drain is shown here:

(slide 11)
So the drain is incorporated in into the pitot-static tube itself.
The buttons are also acting as a drain themselves, for details see Alexander's answer.

Related:

What is the difference between a pitot tube and a pitot-static tube?

To build on Bianfable's great explanation, the "button" drains in the PA28 series of aircraft are typically the lowest point in the system and where any excess moisture/condensation would gather if it gets past the drain in the pitot-static mast. They are spring loaded and when you press them, any trapped moisture or condensation simply leaks out into the cockpit. It's never a large amount so it's barely noticeable.
Care should be taken, however, as to not drain the system (press the buttons) in the air as it will disrupt the pitot-static instruments.

Thanks for the addition, I linked to your answer.


text:
What is the differance between gliding for maximum range and gliding for maximum endurance?
Is there a difference between gliding for maximum range vs. endurance?
Is the glide speed the same for max. endurance and max. range?

No they are different.
Gliding for endurance, maximum time in the air, is at minimum sink rate, which is usually close to stall because it's where maximum lift is being generated. In a glider this is the speed you thermal at.  In a power plane it might be the speed you'd glide at if you wanted to delay the inevitable as long as possible for some reason, say if your engine quit while sightseeing over a lava field during a volcanic eruption.
Gliding for range, flattest glide, is at max L/D speed for the weight, which also affected by the wind's effect on "effective" L/D or glide angle relative to ground covered.   If you were in the volcanic eruption scenario, it's the speed you'd wished you'd flown at as you realize you could have cleared the lava field if you'd sped up to best glide angle.
I'm only partly kidding around; inexperienced glider pilots can fall into a trap, when they get in sinking air, of slowing to min sink to minimize their rate of descent, when the proper action is to speed up to best L/D, or faster, to get through the sinking air as fast as possible, even though their sink rate goes up even more.  I tell students to think of sink as a rain shower while strolling; you run to spend as little time in the rain as possible.

Nice illustration with the lava field!

The lava field should have some pretty good thermals, so yeah you want to be close to min sink to get the most out of them!

Maybe even enough to support a 172... Good point!


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Do thrust reversers only have idle and full modes or are there intermediate settings?
Are there any intermediate thrust reverser settings?
If so, how often do pilots use these intermediate settings?

My experience is on the CRJs but most airliners are similar.  Normally you can work reverse thrust variably between idle and max reverse.  The reverser levers will have a stowed and locked position (all the way down), and there will be a latching device you operate with your fingers to pull them up from stowed.
From there you end up at an idle reverse position (which may have a physical detent you can feel), and there is a range of movement above idle reverse going up to maximum reverse.  You can vary the reverse thrust between idle and max as you wish, although most pilots will just go from stowed to idle to max, then back to idle then stowed on a landing.
One technique issue on using reversers is if you slam the reverser levers back down to fully stowed really quickly, the reverser devices on the engine stow while the fan is still running at high rpm and on some engines you will get a little shot of forward thrust at the end of the landing roll.  It feels like you let off the brakes at bit for a couple seconds.  Once you get a bit of experience, you learn to ease them off to idle reverse and wait at idle a few seconds before stowing.
Reverse thrust is not taken credit for in landing distance calculations, so their use is optional by the landing pilot.  Some airlines may prefer a bit faster brake and tire wear to wear and tear on the reverser system, based on a relative cost analysis, and encourage crews to avoid reverser use unless they need them.
Some pilots will plan not to use reverse but will want it ready just in case, so they will unstow on landing but keep the reverser levers at idle.  This practice is frowned upon by the maintenance types because it counts as an operating cycle of the reverser system (impacting long term maintenance) even though reverse wasn't used to help slow down.

Airbus A320
On the Airbus A320, the reversers are deployed by moving the reverse levers on the thrust levers up. This will result in idle reverse thrust. The amount of reverse thrust can now be continuously set by moving the thrust levers further back towards the MAX REV detent. From the FCOM Power Plant section:

The resulting thrust can be seen in the following chart:

Boeing 737
On the Boeing 737 the thrust reversers are controlled with a similar lever on the main thrust levers, but the main levers are not moving when setting more reverse thrust. You rather pull the reverse levers higher up. From the FCOMv1 (7.20.17 - Engines, APU - Engine System Description):

When reverse thrust is selected, an electro–mechanical lock releases, the isolation
  valve opens and the thrust reverser control valve moves to the deploy position,
  allowing hydraulic pressure to unlock and deploy the reverser system. An
  interlock mechanism restricts movement of the reverse thrust lever until the
  reverser sleeves have approached the deployed position. When either reverser
  sleeve moves from the stowed position, the amber REV indication, located on the
  upper display unit, illuminates. As the thrust reverser reaches the deployed
  position, the REV indication illuminates green and the reverse thrust lever can be
  raised to detent No. 2. This position provides adequate reverse thrust for normal
  operations. When necessary, the reverse thrust lever can be pulled beyond detent
  No. 2, providing maximum reverse thrust.
Downward motion of the reverse thrust lever past detent No. 1 (reverse idle thrust)
  initiates the command to stow the reverser. When the lever reaches the full down
  position, the control valve moves to the stow position allowing hydraulic pressure
  to stow and lock the reverser sleeves. After the thrust reverser is stowed, the
  isolation valve closes and the electro–mechanical lock engages.

Standard Operating Procedures
Regarding the typical operation procedures, I have to disagree a bit with John K's answer. Normal SOP is to always deploy the reversers at least to idle. This is because a high bypass turbofan engine will still produce quite some thrust in forward idle. By deploying the reversers in idle, most of this thrust is re-directed to aid in braking.
Note: On a dry runway, thrust reversers do not have an impact on the amount of deceleration (if autobrakes are used). The more reverse thrust is used, the less will the autobrake system use the wheel brakes resulting in the same total deceleration. Only on slippery runways, where wheel braking is limited by the anti skid system, will reverse thrust result in a shorter landing distance.
How much reverse thrust to use on a dry runway will be airline procedure specific. Typically, when a short turnaround is required, you want the wheel brakes to be rather cool so you can takeoff again quickly. Therefore, more reverse thrust might be necessary before a short turnaround. For the last flight of the day however, brake temperature is of no concern, and idle reverse will be used to save on fuel and reduce noise pollution.


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What is the difference between turbojet and turbofan engines?
What is the difference between a turbojet engine and a turbofan engine?
I know that both of them have a fan and have turbines but how to distinguish them?

Possible duplicate of Why do turbofan engines have low pressure compressors?

This is pretty much a basic definition question - even the Wikipedia article covers it well.

Not a duplicate. The question referred to is based on a misunderstanding of the term Low Pressure Compressor.

What makes you think that "both have fans"? A turbojet does not have a fan.

@Bianfable Turbojets lack bypass fans, but they do have fans.  The compressor stages aren't colloquially referred to as fans, but by definition they are fans.  I wonder if that's where some of the confusion in the question (and comments) stems from.

@CarlKevinson You're right, that's probably where the confusion comes from. Nonetheless, I've never heard the term fan being used for a compressor. See also Compressors, Fans & Blowers – Basic Understanding.

@Bianfable I suspect you have heard the term fan being used for a compressor - Bypass fans are sometimes referred to as the first stage of the compressor.  Fundamentally compressors and fans are both moving gas from low pressure to high.  The only difference is whether it's the movement or pressure that you care more about.  Ultimately though, they're the same thing.

@Carl Kevinson, I have never heard anyone refer to the first stage compressor in a straight turbojet as a "fan".  The term fan is a shortening of "turbofan", just like "jet" was originally a shortening of turbojet.  To a majority of the aviation world the word fan implies bypass.

"How to distinguish them," in practical terms, can be very difficult and may effectively involve consulting literature about the engine itself.

Turbojets and turbofans are very similar indeed:

both are turbine engines;
both create thrust from jet exhaust;
and both have a rotating implement in front that can be called a fan. Although in the case of the turbojet, it isn't called a fan but the compressor first stage.

$\ $ Junkers Jumo 109-004
So what is the difference? There are five types of turbine engines, named after their main means of generating thrust:

Turbojets. The very first type of jet engine as used in the Messerschmitt 262: a compressor, combustion chamber, and a turbine. The turbine extracts enough power from the combustion gases to drive the compressor. All air flows through the engine itself, and thrust is generated by the exhaust flow only. Bypass ratio zero.
Low Bypass Turbine Engines. A bit of a lengthy, non-catchy description. Part of the airflow bypasses the main engine, and is compressed by the first stage of the compressor which has a larger diameter than the main engine tube. The turbine drives the compressor and the (low bypass) fan, exhaust flow from the turbine still generates a sizeable proportion of the thrust - inversely proportional to the bypass ratio, which is 0 < BPR < about 2.
Turbofans. Same as above, named after the large visible fan-like first stage. These engines are high bypass turbine, 5 < BPR < 15 and ever increasing.
Turboprops. Main source of thrust is a propeller, which has longer, fewer blades than a fan to limit drag penalties. The turbine drives the compressor and the propeller, some exhaust flow is still used for thrust generation - according to Torenbeek section 4.3.5, about 5 - 10% of the thrust is generated by the exhaust. BPR of air through propeller/air through engine is up to about 50.
Turboshaft. Main thrust source is a helicopter rotor or other torque requiring device, like an electrical generator in an APU. All net turbine power is applied to the drive shaft, no practical thrust from the exhaust flow.

The five types incrementally use bypass airflow to generate thrust. Increasing bypass air allows for accelerating a larger mass of air at a lower speed, generating the required thrust at a higher efficiency. But bypass air volume is inversely proportional to airspeed: the faster the aircraft, the lower the amount of bypass air that can be utilised. At supersonic speeds, bypass air is very problematic for thrust generation.
The picture below shows the Olympus engine used for propelling Concorde, a pure turbojet with 2 axes and 7 compressor stages per axis.

The GE CF6 with BPR of about 5 depicted below was used for B747, A300 and DC10, and is a turbofan: the first stage of the compressor sticks out over the remaining stages, and bypasses most of the air inflow outside of the main turbojet engine. This bypass air is the main distinguishing feature between turbojet and turbofan. Visually, the large fan is very distinguishable of course.

Thanks for the good answer but i still have a small doubt how to distinguish them in military fighter jet planes like the avro vulcan and the saab jas 39 gripen

@AlexFurnivalKrauss Those are both turbojet aircraft.  Turbofans are much larger than turbojets since they require a large cowl to guide the bypass air.  Nothing supersonic will use a turbofan - they are only effective as subsonic engines.  Military jets tend to use turbojets or low bypass turbofans, even for slower aircraft since the high angular momentum of the fans mean the pilot has much slower throttle response on a high-bypass turbofan vs a turbojet (which can spool up much faster - important in tactical situations, etc).

@Alex, there's little visual difference between a turbojet and a low-bypass turbofan. You'd have to look closely to see if any air bypasses most of the engine, or look it up.

@J... "Nothing supersonic will use a turbofan" is rather confusing when you go on to talk about the low-bypass turbofans that are used in supersonic fighters. (example https://en.wikipedia.org/wiki/Pratt_%26_Whitney_F119) Maybe it's better to say that nothing supersonic uses a high-bypass-ratio turbofan?

@RobinBennett Yes, it is badly written - my fault.  The supersonic Gripen does also use a low bypass turbofan, but on craft like that the bypass ratio is like 0.3:1, which is extremely low.  They're effectively turbojets with a tiny bit of bypass as an efficiency tweak.  So yes - nothing supersonic uses a high-bypass turbofan.

@RobinBennett: Yup, of the ones I looked at, you had to go back before F-14 / F-15 era to find fighters using pure turbojets.  Since then fighters have used very low bypass turbofans.  (Updated my answer with that hopefully-correct fact.)

The main source of thrust is a fan is this true of all turbofans, including low-bypass engines found in supersonic fighters?  Probably not true even with afterburners, and thus not truly the distinguishing characteristic of a turbofan, only of a high bypass turbofan.

@PeterCordes Low Bypass Turbine Engine would be the correct description, but a bit lengthy. The main source of thrust is still the jet exhaust, therefore I classified them with the turbojets.

@Koyovis: normally that would make sense, but in a question about the terminology that doesn't seem like a good idea.

@PeterCordes You're right.

“and both have a rotating implement in front that can be called a fan. Although in the case of the turbojet, it isn't called a fan but the compressor first stage.” Well.....not quite.  Probably better put a turbojet does not have a fan, but rather just has a compressor.  There is no bypass air in a pure turbojet.

@CarloFelicione We seem to be in agreement.

Turbojets/fans don't generate thrust via exhausted jet, the biggest source of thrust doesn't come from the "back of the engine" rather from the front of it like visible in this picture:
https://i.sstatic.net/FdH9Q.jpg

@sophit The total thrust F of the turbojet is $\dot{m} (v_e - v_0) + A_e (p_e - p_0)$, with $v_e $ the exhaust velocity and $p_e$ the exhaust pressure.

@Koyovis: you're right, I didn't write it well... I meant that the picture shows only that the biggest changes in V and P happen in the front of the engine and not in the back. You are obviously free to incorporate the picture in your answer if you will

@sophit Yeah looking at the internal bits of the engine thrust generation is an interesting activity, but in this case not contributing to answering the question in my opinion. The essence being how the net thrust generation is created.

@Koyovis: very interesting indeed  thanks for the answer

In a turbojet, all the air goes through the engine proper, through the combustion chamber and all the stages of compressor and post-combustion turbine blades.
In a turbofan, some of the air is just pushed by a fan around the rest of the engine.  This is the "bypass".  As Harper points out, it's not fundamentally different from a turboprop or extracting other mechanical work from a turbine engine by having the exhaust do more work turning a shaft.

Low vs. high bypass turbofan is about how much of the air goes around the combustion chamber.
In a high-bypass turbofan, almost all the thrust comes from the fan, with turbo-shafts extracting almost all the work from the jet exhaust to power the fan.  The thrust from the hot combustion products coming out the back is minor.
In a low-bypass turbofan, a good fraction of the thrust still comes from the jet part so it's part way between a pure turbojet and a modern high-bypass turbofan.  Better for higher speeds, and faster throttle response without a huge fan to spool up.
A very low bypass turbofan like 0.3:1 is used in the Gripen supersonic fighter as "an efficiency tweak" according to @J...  This is technically a turbofan but performance-wise it's very close to a turbojet, just more fuel efficient at lower speeds and thrusts.  The F-16 uses a turbofan with a 0.71:1 bypass.  These engines can use an afterburner to augment the jet part of their thrust even more.
Early jet fighters often used pure turbojets, but F-14, F-15, F-16, and other fighters in those eras and newer use very-low-bypass turbofans.
Other answers go into more details and comparisons, but remember the key difference is that a turbojet has zero bypass.

I know that both of them have a fan

No, as comments on the question point out, a turbojet doesn't have a fan.

Upvoted for simplicity.  Honest, clean answer without extraneous information.

Turbofans have a bypass, turbojets do not. +1

@MichaelHall: Added some extraneous info, but kept the top of the answer focused on the key point (I hope :)

Completely different design philosophies
They are both turbine engines, and that is where the similarity ends.
In a turbojet, the compressor-burner-turbine package is optimized to make thrust.
A turbofan engine is a type of turboshaft engine.  These use a compressor-burner-turbine core, but use a secondary set of turbine blades to convert its thrust into shaft rotation.  Typically this is on a separate shaft that spins at its own speed. The shaft rotation is used in a huge variety of applications, largely to replace piston engines:

Spin generators at power plants (or aircraft APUs!)
Turn helicopter rotors
Drive pumps
Drive naval screws (water propellors)
Spin air screws (airplane propellors; this is called a turboprop engine)
Spin prop-fans
Spin a gigantic, outsize ducted fan.  This is called a "turbofan" engine, and this is what you are asking about.

But these are all the same thing, really: a turboshaft engine rotationally driving some sort of thrust maker. (in the top instance, electron thrust, but let's not quibble).
The mere fact that the shaft rotation comes from a compressor-burner-turbine engine is mere coincidence.  It would be perfectly possible to have a diesel engine spin a ducted fan, if you could get a diesel compact and powerful enough.
Get it?  The ducted fan may have a cosmetic resemblance to the intake of a turbojet engine, but it's simply used because it's the best "prop/fan" for the job.  If turboprops or actual propfans were more efficient, they'd be used instead.
Of course, the secondary turbine stage could be intentionally made less efficient so it fails to convert all the thrust to shaft rotation.  In that case, some thrust would still occur in turbojet fashion.  That's actually a "knob" the engine designer can turn.  In helicopters, they peg it all the way to "shaft", and you see where many helicopters put a 60 degree bend on their turbine exhaust. A stationary generator also has zero use for thrust except to get the exhaust plume to move along. For most civil turbofans, it's inconsequential; the thrust reversers don't even bother reversing jet thrust and only reverse fan thrust.

+1 You could also mention another ubiquitous application of turboshafts: the APU.

In my understanding, and according to everything I've read about the subject, turbofans are not turboshafts. In a turbofan a significant amount of the thrust comes from the turbine exhaust (depending on the bypass ratio), whereas in a turboshaft the exhaust has no (signifcant) thrust. A turboprop, for example, is indeed a turboshaft connected to a propellor. As a comparison, modern high-bypass turbofans have bypass ratios of up to 12:1, while turboprops have bypass ratios of 50-100.

@RoelSchroeven I discuss hybrid partial thrust engines in my last paragraph. Note how modern civil turbofans don't even bother thrust reversing the turbojet section...

@Harper: what you call "hybrid partial thrust engines" are exactly what other sources call "turbofans". Engine designers could increase the bypass ratio even more, reducing the thrust of the jet airflow to almost nothing, and the result would be what you describe. But they don't, creating a fundamental difference between turbofans and turboshafts. I can't get rid of the impression that you're using terminology differently from everything I've read before.

@RoelSchroeven because what you've read before is based on the incremental baby-steps into turbofans, like those used on the 707. I am talking about an entirely modern sense of turbofans, where essentially all the thrust comes from the fan, and those old 707 turbofans are a dialing back/hybrid. These are different paths of thought arriving at the same destination.

After reading all the answers I felt that none of them really explained the answer in a way understandable to a layperson, so I will attempt to do so.
First of all, both types of engines will burn fuel to generate energy, which is ultimately used to accelerate a stream of air towards the rear of the aircraft to create thrust. They differ in the method by which they accelerate that stream of air. Newton's third law (roughly) states that for every action there is an equal and opposite reaction, i.e. if the engine pushes the air backward, the engine (and the aircraft it is attached to) must accelerate forward. This is what is meant by the word thrust. Note also that fuel needs an oxidizer to burn, and that oxygen comes from the air.
Turbofan
A turbofan is akin to the classic and intuitive propeller engine. The fan blades push the air just as a fan does in your house, and the engine housing directs that airstream backwards. Some of the air must also enter the engine core to oxidize the fuel in order to generate the energy which drives the propeller shaft, but most of it goes around the core and out the back. In essence, the fan blades are "pushing off" of the air, as you might push yourself off the wall of a swimming pool to accelerate your body through the water (or better, just as paddling in water pushes you forward while pushing the water backward). Note that the engine exhaust here does not play a direct role in generating thrust, the thrust comes from the large fan blades pushing air backward.
As an analogy, consider a car: Fuel is burned in the engine by mixing it with oxygen from the air (often using a turbocharger which compresses air in order to burn the fuel faster, which the turbofan engine also uses); the burning of fuel pushes the pistons which then turn the drive (propeller) shaft; the drive (propeller) shaft then turns the wheels (fan blades) which push off of the pavement (air around the fan blades) in order to push the car (plane) forward. Note again that the exhaust from your car doesn't play a direct role in pushing the car forward, it's just the byproduct of the burning of fuel.
Turbojet
A turbojet, on the other hand, is exactly as its name implies, a jet. It directs all of the air into the engine core, mixes it with fuel and compresses it to achieve high temperatures and a large pressure gradient, rapidly accelerating the air and shooting it out the back like a rocket. The physics is akin to covering the end of a garden hose with your thumb to increase the speed of the stream of water. In this case, however, you are also mixing it with fuel to raise its temperature and increase the speed even further. Thrust is then created by ejecting the high-temperature, high-speed exhaust of the engine backward, which is akin to the way that a rocket works. The difference is that a turbojet uses compressed air as the oxidizer while a rocket has to carry its own oxidizer (since there is no air in space!). Note the engine still has fan blades that are needed to push the air back and compress it. Contrast this to a ramjet, which is an engine that operates on the same principle but only works at extremely high speed where the sheer pressure of the incoming air is enough to run the engine without the assistance of fans. In any case, the word "jet" implies that the thrust is generated by a stream of high-speed exhaust directed out the back of the engine, i.e. the exhaust plays the primary role of what is being ejected to generate thrust.
In case it isn't clear, the prefix "turbo" is referring to the compression stage. Compressed air has a higher density of oxygen and will burn the fuel faster and more completely than uncompressed air. At high altitude, compression is necessary since the density of the air is so low there.
To wit
One more condensed explanation of all the above using the analogy of the internal combustion engine with a turbocharger. In the internal combustion engine, the turbocharger compresses the air, which is then mixed with fuel and injected into the combustion chamber. The piston compresses the mixture further and it is then ignited. The ignition of the fuel causes the gas to rapidly heat up and expand, and that pressure is used to push the piston back which turns the crankshaft and thereby turns the wheel.
Now to the turbofan and turbojet. Both engines have a core that compresses incoming air, mixes it with fuel, and combusts it. The difference is that a turbofan uses the resulting pressure of the expanding gas to rotate a crankshaft which turns the large fan blades to generate thrust. A turbojet, instead of using the pressure to turn a crankshaft, simply allows it to escape out the back end of the engine thereby generating forward thrust.
Hopefully this clarifies some of the other answers.

This isn't really that concise.  You mixed in asides about several other types of engines and the basic physics of propulsion, effectively answering many questions that weren't asked.

@ErinAnne I went ahead and edited out the word concise, which I put in before writing the entire answer. I felt the other answers are significantly lacking in being understandable to someone who does not already understand the physics of propulsion. Given the paucity of information in the question as to the knowledge of the asker and the extremely general title, it was worth including the relevant background information so that anyone who wants to know the difference can learn something. It's also a hot network question so there may be people coming here with no background.

A burning turbine consists of compressor stage, burning stage and gas turbine stage. Both compressor and turbine stages consist of sets of stator and rotor blades and the rotors are connected via shaft so part of the work the turbine generates can be used to compress the intake air. The output from the turbine stage is a high-velocity jet of hot air-fuel-burning products mixture (resulting in exhaust thrust) and a revolving shaft (carrying torque). All turbine motors share this base.
The differences between engines is their optimisations and thus designs. Turbojet engines are optimised to maximum exhaust thrust and minimum torque securing optimal work of the engine.
Turboprop, Turbofan and other Turbo- engines are optimised to produce maximum torque on the shaft and the exhaust thrust is of lesser interest here. The propeller then produces the main thrust of the whole engine. Turbofan engines may be considered advanced -prop engines because they optimise the airflow through the bypass resulting lower losses caused by air escaping the propeller crossection. And the propeller is called fan because we need to distinguish the designs, right?

Turbojet - Narrow, long, optimised to jet-generated thrust. Used mostly in fighter aircraft and mounted in-hull.
Turbofan - Wide, short, optimised to propeller-generated thrust. Used in airliners and bombers. Wing-mounted  or mounted out-of-body.

As you can see in Kyovis' images, when looking at the front of the engine, the thing you can see on the Turbojet engine are the blades of the first compressor stage. On the Turbofan engine you can see the blades of the propeller - here called fan -, the compressor blades are much smaller and are hiden behind the large propeller.
In other words, the engines consists of blades, shaft(s) and casings (neglecting fuel supply and controllers). The blades are:

compressor rotor blades - their purpose is to compress the incoming air to pressures and temperatures optimal for combustion,
compressor stator blades - their purpose is to optimise the airflow through compressor increasing its efficiency,
turbine rotor blades - their purpose is to convert thermal energy of the exhaust gases to mechanical work as the torque on the shaft,
turbine stator blades - their purpose is to optimise the airflow through turbine,
propeller blades - their purpose is to convert torque from the shaft to thrust. Present only on Turboprop engines,
fan blades - same as propeller blades but enclosed in bypass chanel. Present only on Turbofan engines.

The interior/exterior mounting really isn't true.  There are internal turbofans and external turbojets, as noted in other answers and comments on the question.

I'll make it really simple
Turbofan engines have a core and a bypass duct. 83% of the thrust is produced by the bypass air whereas the other 17% is produced by the core. The bypass air is simply cold air which is propelled rearward by the huge fans on the front of the engine. The rest of the air enters the core where it goes through the compressors, combustion chamber and exits out of the exhaust with high velocity- producing thrust.
Turbojet is basically a turbofan with NO BYPASS DUCT. The only source of thrust is the core. That's it! I'll provide you with 2 images below so that you can understand better.

100% fans like most big passenger transport turn as much heat as possible into mechanical power to turn the fan that pushes the plane along.
Lowest noise best energy conversion.
Turbo jets convert heat into high pressure exhaust to produce reaction thrust like a rocket to enable fighter jets to break the sound barrier. Not so good thermal efficiency and very noisy.
Late 1960's engines started bypassing some of the intake air past the power producing part of the engine to mix with the hot gas in the tailpipe come from the core power generator to create expansion of the cooler by passing air and hence raise thermal efficiency.
So that's really the 3 types of jet engine.
Hot high pressure for pure light weight supersonic thrust. (20:1 pressure change),High bypass jet for some reaction but with an efficient cruze option (8:1) pressure difference = virtually all modern multirole combat planes. Total mechanical conversion (1.75:1 pressure change) = virtually all modern passenger aircraft.

"Total mechanical conversion" sounds more like a turboprop, not turbojet, so saying that's what "virtually all modern passenger aircraft" use is wrong. They are using high bypass turbofans.

And what is a 100% fan?

The turbojet engine develops most the thrust in the exhaust nozzle.
The turbofan engine develops most of the thrust in the fan.
No engine develops all thrust in the exhaust or in the fan. There is a balance between the two components
Exception makes the turbo shaft where the turbine absorbs all power from exhaust gas to drive the shaft. Both airplane and helicopter turboprops produce no thrust in the exhaust. Turbo shaft engines are also used in ground applications ( power drivers) and naval applications for military because of their huge specific power(power to weight ratio).
Note: the name turbo shaft relates to the engine by specifying the power is at the shaft. The name turboprop refers to the turboshaft engine plus the propeller assembly.

A turbojet has no fan so the core produces all of the thrust. A low bypass turbofan will still produce most of the thrust from the core. Also, turboshafts still generate some amount of thrust from the exhaust.

You’re correct, turbojet has no fan stage. However it has axial compressor and a consistent fraction of the thrust is developed in the compressor. As for the turboshaft, this name is specifically used for a turbojet where the turbine extracts all the power and there is no remaining thrust in the exhaust. If you know of any turboshaft engine that produces thrust in the exhaust nozzle, then please let me know.

@WindSoul, there are several misleading little inaccuracies in your answer and comment.  A turbojet gets 100% of its thrust from the exhaust nozzle.  Period.  All gas compressed up front MUST exit through the exhaust after it has burned.  The only way for compressed gas from the front section to produce thrust and not exit through the exhaust would be if you allowed some of it to escape.   If this air did “bypass” the hot section, it could be expelled rearward to produce thrust.  There are engines that do this, they just aren’t called turbojets.  They are, (pause…) called turbofans!

Also, be careful you don’t mix names.  A “turboshaft” is not a “turbojet” that turns a shaft, it is a gas turbine engine that turns a shaft.  ("shaft", "jet", "prop", and "fan" all describe the part that receives the output of the engine, and does the actual work.)  And finally, be careful of absolutes.  You may say that 100% of the force turning the shaft comes from the gas driven turbine, but you cannot say that 100% of the engine’s power is extracted by the shaft.  Sure, it doesn’t produce any useful thrust, but there will always be some energy lost to inefficiency.

P.S.  Did this question really need another answer?!


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How to keep an aircraft stationary with an inoperative parking brake?
After High Speed Rejected T/O and an imminent evacuation, how would a pilot keep the airplane stationary with an INOPERATIVE parking brake?

High speed rejected take off doesn't usually mean evacuation... What else is happening?

Turn off the engine(s), wait for someone to come out with wheel chocks.

Also why do you assume that the parking brake would be inop? RTO's at V1 are part of the certification process, which is brakes at "most worn" and no use of thrust reversers allowed. The brakes still work (unless there is something else going on). I think this question is based on a false premise.

Keep pushing hard on the wheel brakes... And hope those do work, obviously.
If there's soggy ground next to the runway, let the aircraft roll onto that and the wheels should bog down into the mud.
There's precious little else you can do unless you want to deliberate drive the aircraft into something solid.

Who is going to volunteer to stay on the plane to keep their feet on the brakes while everyone else evacuates?

The pilot with the biggest fooot should stay.  ;)

Would that big-footed person really push so hard? I mean, in a car it's quite normal to keep pushing the wheel brakes instead of using the parking brake. At traffic lights, for example. After five minutes that might get annoying, but that's it. Basically the foot is resting on the pedal, not really pressing it. Except on a hill, but that would be rather unusual for a runway.

@PerlDuck  I've never heard of a parking brake that "just failed." It could, but this is way, way far off in the realm of the hypothetical... high speed abort, followed by "hey, the parking brake isn't working." With engines off (part of evacuation checklist) & light wind, it isn't rolling anyway. In high winds or significant slope... yeah, maybe somebody sits there holding the brakes until somebody shows up with a chock. Highly, highly improbable.


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What will happen if suddenly a window breaks during flight?
What will happen when a window of a flying aircraft breaks and is removed? Will we survive in that aircraft?

Do you mean the window of a light aircraft flying unpressurized, or an airliner which is pressurized?

Aloha 243 lost part of the cabin roof and landed with just one fatality.

This has to be a duplicate, but I can't find a good target...

If a passenger window blows out, there'll be a bang, a whoosh for a couple seconds as the air pressure escapes (it's just a bigger version of cracking open a bottle of soda), fog will form in the cabin, and it'll get quite chilly from the sudden pressure drop. Your eardrums will feel like they also want to blow out for a second, they might hurt, and you may notice the pressure differential in your sinuses until that air escapes.  You might feel abdominal bloating if you're paying attention in all the commotion.
Whoever was right at the window, if their frame is small enough to be forced through the hole, might disappear out it (mostly, if their plevis will fit through).  Or if they're big enough, they might just form a plug in the window frame and help things out, (although it'll probably kill them if their head made it outside).
You don't want to be too close to the window, but otherwise you'll be fine if you get the drop down mask on in time.  If the plane is in the upper flight levels, you'll have maybe 15 seconds before you pass out, after which someone else needs to put it on for you, or you could develop brain damage or even die.
Just sitting stationary, you won't "feel' the thin air just breathing it and start gasping for breath unless you're exerting yourself (a surprising discovery for me when I went on a high altitude indoctrination course with a chamber ride long ago).  The crew will immediately dive the plane to get to thicker air, so you'll be at a safer altitude pretty quickly, within a minute or two.
The passenger windows are normally 1/4 inch thick acrylic plastic, and based on the tensile strength of acrylic (about 8000 psi IIRC), the strength of the window is more or less the same as the surrounding, much thinner, aluminum skin, so it takes quite a lot to make one blow out, which is why the event is so rare.

Thank you @john-k, your answer is very helpful.

There are some mistakes here.  An emergency descent is not a dive.  https://aviation.stackexchange.com/questions/80437/what-pitch-attitudes-are-common-during-an-emergency-descent-for-an-airliner

If you followed the instruction to wear a seatbelt, you are not getting sucked out the window.

"you could develop brain damage or even die."  Why do you believe the time an emergency descent takes is long enough for brain damage to occur?  Athletes regularly go to FL290 and then walk back down.  Most of them survive.

@AnonymousPhysicist, to most passengers an emergency descent will feel like a dive.  And those who climb Everest without O2 do lose some brain cells in the "death zone" above 25K.

I've done emer descents in the CRJ during recurrent training.  You feel like you're diving. and so will the pax.  The cabin will be a massive roar from both the open window and the speed at Vmo.  Pax may or may not be wearing seat belts. Ppl usually put them on loosely in cruise, and you will still get sucked out if you are small enough to fit through the window frame. Exposure at FL390 or 400, you could potentially get brain damage from oxygen deprivation by the time you are below FL250.  Most ppl aren't athletes and if you're a smoker, you're in bigger trouble if nobody gets a mask on you.

@JohnK Your own answer in the linked question says 10 to 15 degrees.  That is not a dive.

It's divey enough.  It just a conversational answer. I'm not describing a Stuka attack.

In 1990 the British Airways BAC One-Eleven captain of a flight from Birmingham (UK) to Malaga (Spain), Timothy Lancaster, got sucked out when a cockpit window blew out. The wrong bolts had been fitted by a maintenance worker. This happened after the aircraft had reached 17,300 feet 13 minutes after take-off. Another crew member, cabin attendant Nigel Ogden, shortly assisted by others, held onto him by his ankles until they landed.
They thought he was dead until they landed at Southampton. It was considered essential to hold onto him if possible in case he caused damage by hitting the left engine, wing, or horizontal stabiliser. He survived with frostbite, bruising, shock, and fractures to his right arm, left thumb, and right wrist. He returned to work after less than five months. Ogden suffered frostbite and a dislocated shoulder.
More recently, in 2018, a First Officer was partially sucked out of a Sichuan Airlines Airbus A319, at 30,000 feet, this time due to a faulty seal. He also survived.
Others have not been so lucky.
British Airways Flight 5390 (Wikipedia)
Sichuan Airlines Flight 8633 (Wikipedia)
Airliner accidents and incidents involving in-flight depressurization (Wikipedia)

By Lancaster's hospital bed, Nigel Ogden has his arm around cabin attendant Sue Prince.

The most common window issue for commercial airliners is a cracked windshield.  They happen all the time.  In this case the window is "broken" in the sense that you need to replace it; often it is difficult to see through the window, too.

AAIRB Window Picture
Usually, the flight lands quickly.  The window is replaced.  Then the plane returns to service.  It's not very exciting.
A broken window is not usually "removed" because the window has multiple layers.  Only under very rare circumstances does more than one layer break.


text:
What operation is this aircraft performing?
While reading Wikipedia about Ust-Kut Airport, I've run into the following picture:

I'd like to know which operation is depicted here?
This doesn't seems to be a take-off, as per my very limited knowledge. It is by far landing. If I am correct then how it is possible that a landing plane, at this altitude, still doesn't have its gear down?

It looks to me like a demonstration flight. I am not sure where to check whether there was any airshow on 8th December 2009 there.

It could just be a random low pass. There may never have been an intention to land. Or, they may have been trying to gauge the condition of the runway from closer than traffic pattern altitude. I’ve done low passes to practice tracking the runway and to scare wildlife off of the runway. Or, even just to set myself up for a missed approach during multiple practice instrument approaches. It cuts down on time.

On what basis do you rule out the photo being taken immediately after the gear retraction after takeoff? Seems like the easiest explanation to me (though the low pass option, for demo or runway inspection purposes are possible too... but I'd have my gear down for an inspection pass over the runway).

The aircraft has no flaps deployed which suggests neither take off nor landing, but a low pass. Why it's doing this would be a matter of speculation.

Well, definitely not a normal takeoff. In such case, by the time the gears have fully retracted, the plane would be several hundred feet high.

@Jpe61 -- not if you keep the nose down & the gear retracts quickly.

Well that would not be a normal takeoff.

@RalphJ Jets normally climb away with a much higher pitch angle than that. At any rate, the clean config (as well as the attitude) say it's just a low approach.

That happens to be a fantastic way to scare ungulates off of the runway.

The flight in question is most probably an evaluation/calibration flight for radio navigation systems at Ust-Kut. The aircraft is a Yak-40 special version designated for such tasks:
Aviamuseum - Yak-40 "Calibrator"

Wow, nice. Were you able to make out the tail number from the picture, or did you just recognize the paintjob and double-check from there?

@T.J.Crowder luck and mad googling skilz  Of some help was the unusual antenna arrangement, and understanding of what such buzzing might be about. The paint scheme as such was not sufficient, as it is carried by some Yak factory a/c.

It looks like a low pass, for unknown reasons but more likely a demonstration of some kind than an runway inspection. That's just a guess, of course, but I think it's the simplest thing that explains what the picture seems to show:

Gear retracted means it isn't landing
Level flight means it isn't taking off (probably)
Very low altitude means it isn't asking the tower to check if the gear is down (that sometimes happens if there's a gear problem and the pilots don't know if it's down or not)
No flaps means it isn't trying to fly slowly (which would be better for a runway inspection)
Directly over the runway (apparently) means it isn't doing a runway inspection (flying off to one side would give a better view of the runway)

Having said all that, the angle of the picture doesn't clearly show the aircraft's position relative to the runway. And I know nothing about turbine aircraft operations :-) So I may be completely wrong.


text:
What are the pros and cons of sealing the gap between the wing and the aileron?
I read somewhere that sometimes gap between the wing and aileron is sealed and this is done to reduce drag. But somewhere else I read that "slotted" ailerons allows for better aileron effectiveness. So which design is beneficial? Is it possible to find an optimum solution or one design exclude other one? And second question - which type of drag unsealed ailerons generate?

The question title had me imagine a situation which is definitely a disadvantage to sustained flight but a unique experience for the marine animal.

I immediately thought, "Doesn't the weight of the seal make each aileron hard to move?  And don't you need seals with identical weights on each aileron to balance them out?  What if one seal dives off the aileron into the water before the other?"

This will become a long answer again.
The most important benefit of an internally sealed aileron is reduced control forces. Ailerons require lots of attention to keep forces small while still avoiding a negative force gradient (that is when the aileron runs into its stops all by itself). In most manually controlled airplanes the ailerons become stiff as a brick at high speed and the designers of airplanes like the P-47 and the Do-335 even used telescopic sticks which would give the pilot a longer lever arm at high speed, albeit at the price of a smaller deflection range. But the larger deflections were impossible to steer anyway …
Only when much care is put into aileron design will control forces be manageable at high speed. Examples are the Supermarine Spitfire or the Junkers Ju-88. To be able to control the plane with only two fingers on the yoke at 500 km/h required several hours of flight testing and adjustable aileron hinges in case of the Ju-88. Another fine design is the English Electric Canberra (known as the Martin B-57 to you Yanks) which had a spring between stick and ailerons. The pilot would only move one side of the spring while the other would move the control rod running to the ailerons. On the ground, you could move the yoke easily ±60° while someone else would keep the aileron from moving - that is how soft that spring was. Now you have a moving mass and a spring - what comes to mind? Flutter! This was a very risky design and it shows how desperate the engineers were to get the aileron forces down. And of course those ailerons were sealed. With an internal seal.
You can either use a brush seal or a cloth seal, but in all cases you need something protruding forward of the hinge line into a cavity inside the rear wing. This restriction will keep maximum deflections down to less than ±20° but will produce the best aerodynamic balance. In case of the Canberra, the chord of this forward plate was almost 40% of the aileron chord, measured from hinge to trailing edge. This is what it takes to compensate >90% of the aerodynamic force. The F-86 Sabre ailerons were similar and used a cloth seal while the Canberra used a brush seal (IIRC). The downside is the maintenance and care to keep the cavity clean. Especially water trapped there which freezes as the airplane climbs into colder air can produce unpleasant surprises.
If you only fly slowly, an external seal is also possible. This is popular on gliders where sometimes the hinge line is placed as closely as possible to the lower skin in order to seal the aileron gap with a sticky tape. More elaborate seals use a thin metal ribbon like in the picture below on a PA-28. The main benefit is a higher aileron effectiveness and a very slight drag reduction. The downside is a bit more friction and more care to keep the seal in shape.

Exernal seal (picture source). I would have preferred to show that of the ASW-20C here, but could not find a decent picture online. The ASW-20C tries to extend the laminar run on the lower side across the flap and aileron hinge line with its seal.
External seals only improve the aerodynamics and aileron effectiveness, but don't help with the stick forces. Since only one side can be the one with the hinge, the opposite side still has a gap.
So why would you prefer to have a vented aileron? First, this allows for higher deflection angles. Second, now the aileron nose can be shaped such that it helps to tailor the stick forces. Case in point: The Frise aileron.

Frise aileron (picture source)
At very small deflection angles the nose of the aileron stays within the airfoil contour. This allows for a stiff centering of the ailerons, so you can feel when the ailerons are neutral. Note that a Frise aileron should be combined with aileron trim! When the deflection increases, the nose of the trailing-edge-up aileron will poke out of the contour and produce a suction peak which forces the aileron to an increased deflection. This force now is transmitted via the linkage to the opposite aileron and helps to force it trailing-edge down. This means at moderate deflection the stick forces become pleasantly low. Once the aileron deflection is too high for the flow to stay attached to the aileron's suction side, the pressure distribution over the aileron changes, the forces increase rapidly and the stick forces tell the pilot that he has reached the limit of the useable range. At high speed this helps to prevent overstressing the wing while at low speed the aileron can be deflected further, maximizing its effectiveness. The picture above mentions drag - this helps to reduce adverse yaw a bit, but the main reason for a Frise-type aileron should be lower mid-range stick forces.
Why else would a vented aileron be preferable? The gap reduces aileron effectiveness but keeps the flow attached at higher deflections. This gives more control authority at the price of increased drag. But there can be more: A clever way to install vortex generators can be found on the Short Belfast:

Tail of the Short Belfast from below (own work). Note the vortex generators hidden in the hinge gap - they only move out into the airflow when they are needed: When the elevator is moved to large negative deflections. This is not an aileron, admittedly, but the same can be done at the aileron, too.
To summarize:
Pros of internal seals:

much lower control forces at high speed
good aileron effectivity

Cons of internal seals:

limited deflection range
have to be kept clean all the time

Pros of external seals:

No gap on one side, lower drag
good aileron effectivity

Cons of external seals:

a bit more friction
care is needed when handling the delicate seals

Pros of vented ailerons

allow to tailor stick forces and reduce adverse yaw
higher deflection angles possible, especially at low speed

Cons of vented ailerons

more drag
lower effectivesss

The key factor is the airflow through the gap in front of the aileron itself. This gap increases the maximum lift coefficient (and thus effectiveness) of the aileron by introducing high-energy airflow from the pressure side to the suction side of the wing, thereby delaying separation. Without the gap, the adverse pressure gradient on the suction side would be too great and the flow would slow down and separate from the aileron.
This is the idea behind F1 cars having multi-element (up to 6) front wings to maximize the lift coefficient and thus lift of a relatively small wing.
OTOH, the flow through the gap introduces parasitic losses by changing the direction on the suction side airflow, while also increasing the thickness of the boundary layer.
In the end, the tradeoff is between a smaller and lighter aileron and lower parasitic drag. For a STOL aircraft design, weight is paramount, so it pays to lose a few knots of cruise airspeed to have smaller and/or more effective ailerons, which could also work as flaps (flaperons). On a sleek twin-prop aircraft, however, speed is a major selling point, adding a few tens of kilos to the aircraft and lengthening the takeoff ground roll by a few tens of metres is not a big deal compared to outright speed and fuel efficiency.

You'll actually see airplanes with "speed kits" that have the gap seals added to cover the slot to stop the leakage for the drag reduction.  Overall, the slot isn't worth it unless you don't want to go very fast.


text:
What kind of weather conditions would produce turbulence with an intensity of 90 feet per second?
This is the official documentation that we found regarding current commercial airplane design criteria; https://www.law.cornell.edu/cfr/text/14/25.341
It says the criteria for continuous turbulence is 90 feet per second, or 'fps'. Is this a big improvement from the criteria of the 1950's and 1960's, which saw several commercial airplanes lost due to turbulence-induced structural failure?

90fps approximates to 120kph or 75mph, so winds of that strength aren’t uncommon.  The question of turbulence is rather different because it presumably refers to a localised airspeed relative to the airframe.  Like many test criteria this is easy to set up in a wind tunnel but it’s difficult to predict what real-world atmospheric conditions would result in that rate of air movement.

Thunderstorm, bad day in the mountains, strong frontal boundary, microburst.

90 feet per second is 5,400 feet per minute. Per the PHAK, Chapter 12 (p.12-11), a microburst can produce those conditions:

A typical microburst has a horizontal diameter of 1–2 miles and a
nominal depth of 1,000 feet. The lifespan of a microburst is about
5–15 minutes during which time it can produce downdrafts of up to
6,000 feet per minute (fpm) and headwind losses of 30–90 knots,
seriously degrading performance.


text:
Can I do an alternator check at idle in a C152?
I have one question about alternator check in Cessna 152. During engine run-up we should check whether alternator is working or not, so we switch it off and then the warning light should illuminate. But we do it on pretty high RPM (1700). Would this warning light illuminate if I switch off the alternator also on lower RPM i.e. idle?

The problem is the light might come on on its own at idle, which is normal.
The warning light comes on when the system voltage is below 12.5 volts on a 14 volt system, and 25 volts on a 28 volt system. Automotive alternators (which most Lycomimg powered light aircraft have) need to spin at at least 2000 rpm to make adequate charging current to maintain system voltage under load.
The belt drive on the engine that overdrives the alternator relative to engine rpm may or may not maintain this rpm with the engine idling, so the bus voltage of the system may be just over 12.5/25 vdc or just below it, depending on the loads on the system (like radios and lights, etc.) and the alternator's condition.
The 152's POH specifically warns you that the light may come on after extended idle periods with electrical loads on the system, so this is to be expected, and it means that a test done with the switch can't be trusted to give a valid result (the light could be on because of "normal" idle low voltage, not because you switched it off).
Doing it at runup rpm just makes sure the alternator is spinning well above this marginal operating condition so it's producing normal charging current when you switch it off.

Ok, I understand. One more question: so there is higher chance that the alternator light will illluminate "by itself" if the airplane is at idle for a long time; and there is almost no possibility, that the light won't illuminate if the alternator is switched off at idle engine RPM (assuming no warning system failure)? Because some time ago someone told me that the light won't illuminate if I switch the alternator off at engine RPM close to idle, because there is no load on it, what I assume is false.

I believe so.  All the light knows is there is less than 12.5 v at the bus.  Whether that's because you turned the alternator off by switching off its field supply with the alt side of the master, or because it's current output isn't enough to keep the voltage over 12.5, shouldn't matter to the light itself, so it should come on if it's off at idle, then you switch off the alt.  Maybe they mean if you turn the ALT and BAT sides off together you won't get a light.  Anyway, try it yourself and see what happens.

One more question, voltage in C152 electrical system is 24 V, so system allows voltage drop to 12,5 V? That's a lot imo.

Oops you caught me out. I was talking about 14v systems and forgot that the Cessna changed their airplanes to 28v systems in the 80s I believe.  I see that both 12 and 24 v batteries are available for 152s so it looks like the change was made sometime in mid production (with the 152 II?) but it's hard to find any info on what year the change was made.  150s were 14 v though.   In any case, if the airplane has a 28 v system, the light would go on at 25v (12.5 x 2).  Otherwise, same same.  Note the battery voltage is 12/24, but the normal system voltage maintained by the alt is 14/28.

BTW the principle advantage of upgrading to 28 v is a tiny weight reduction because lighter thinner wires can be used to carry the same wattage, and a bit better electrical reliability (in theory) because the higher voltage makes circuits a bit more tolerant of degraded electrical connections.  Operators were not too happy with the switch because if you had a fleet of 14 v airplanes you now had to also stock parts for 28 v systems if you acquired some 28 v ones.


text:
Can planes go to space?
What are the reasons why planes can't go into space?
https://thehumble.co.uk/aircraft-with-forward-swept-wings/

What does a forward-swept aircraft wing have to do with your question? Can you define "space"? Altitude higher than the Karman line?

Closely related

Err... Have you forgotten the Space Shuttle?  Or the Air Force's X37b?

They can, and have. So many of them actually. Especially into space, not orbit. You have rocket powered ones like X15 and space shuttle. Rocket carried ones like X37 and Buran, jet powered ones like Mig-31.

the space shuttle was just a glider, right ?

Dilshan, note that propellor aircraft certainly could not go to space.

Dilshan, if you're still reading and want an extremely simple one-sentence answer, it's that: "Wings, and airplane motors, only work in the air."  That's it.  In the same way a "boat" couldn't work in space since "keels only work in water".

@Fattie: No, the Space Shuttle took off under power, and glided to a landing.  Which is essentially what you do in most small planes - pull the power back to idle somewhere on base or final.  Besides, gliders are airplanes.  Launching something like the X37b (if it doesn't actually have its own engines) is no different, in principle, from getting a tow to launch your sailplane.

@jamesqf , I think it's a stretch to say it "took off under power".  it was just an object attached to a space rocket, as it were!  heh!  it just had utterly no connection to "aeronautics", "airplanes" when taking off, I think!  :)  I think the point is for the OP, the "space shuttle" certainly didn't "fly ["in any way like an airplane"] in space".

@Fattie: On the contrary, the Shuttle had its own engines,  It used a droppable fuel tank and rocket-assisted takeoff, both of which have been used by military airplanes since WWII.  Likewise, there are airplanes that can do vertical takeoffs (the Harrier, the Convair Pogo: https://en.wikipedia.org/wiki/Convair_XFY_Pogo ) and vertical climbs under power.

Basically it's because they have engines that need atmospheric oxygen for thrust.  Stick a rocket motor on it, with enough power to achieve escape velocity, and provide a self contained compressed gas source for pressurization at say 10 or 15 thousand feet cabin altitude and make the pressure hull fully air tight (airplanes normally leak quite a lot), and away you go to the stars if you like.
Of course, now you have to get back down without burning up so you need maneuvering thrusters to control attitude on re-entry, and you need some kind of heat shield...
After working all that out, voila! You end up with the Space Shuttle.

Oddly enough the Buran had air-breathing jet engines.

Wasn't that for landing or something?

Yeah, it was only to be used during reentry and landing.

The rest of the rocket was just there to carry the fuel needed to get the space shuttle up there.

Just for the record, I feel the "pressurized" bit is pretty irrelevant, right?  A number of aircraft (military etc) are not pressurized right?  The dudes have to wear spacesuits.  Actually, was the Vostok and other early craft pressurized or did they have to wear suits exclusively??

They were pressurized but they wore space suits in case.  If you run at 8 psi which most airliners run at, that gives you a 15000 ft cabin in space which most people can tolerate, more so if the oxygen content is increased.

There are two major problems with trying to take an aeroplane to space.

The engines are air breathing, this makes them far more efficient than rocket engines, but means they only work over a relatively narrow range of speeds and atmospheric densities.
Planes are designed with wings to use aerodynamic lift, this is far more efficient than directing thrust downwards, but again a given set of wings is only effective over a relatively narrow range of speeds and atmospheric densities.

There is a big gap between the highest speed/thinnest atmosphere at which aerodynamic flight with air breathing engines is practical and the lowest speed/thickest atmosphere at which orbit is practical. So-far the only successful way to get into orbit has been to brute-force through with big multi-stage rockets.


text:
Why photographing airports is banned if everything is perfectly pictured in Google Maps?
While staying at Premier Inn, next to Abu Dhabi International Airport (AUH/OMAA), I was more than surprised to find out warning signs that I'm strictly prohibited from taking photos of the nearby airport form the hotel's roof pool.
Can someone explain me, why such regulations are still in place or how do they aid to airport security, if everything (what I'm banned from photographing from this distance) is perfectly pictured, in a far better details, in Google Maps?

I'm voting to close this question as off-topic because this is better asked on Travel.SE.  It has little to do with aviation (the same is true of oil refineries for example) and much to do with culture and history.

If you're so certain that this fits better for Travel.SE then why you don't flag it for a migration instead of closing? This is the second time this happens to me on this site. A bit weird. I was told that migration is far better across SE than closing questions and asking them again on the other site.

BTW: I'm completely lost, what -- in your opinion -- question about security and regulations has to do with culture or history? Did you make some strange copy-paste from another question?

This would almost certainly be off-topic on Travel. Why are such regulations in place? Because the government decided, if it's the law, or the hotel decided, if it's their policy. We don't know why the government or hotel decided to do such an illogical thing. (Well, it might make sense for the hotel: a crowd of people with cameras around the pool might be off-putting for people who want to use the pool. But that would be pure speculation.)

I wouldn't want to put words into @Simon 's mouth but it's a cultural question because some jurisdictions choose to prohibit taking photos of planes and airports, whereas others have no problem with it. Some cultures are fine with people taking photographs of publicly visible things; others view it as some kind of problem.

I can't flag for migration, otherwise I would. In my opinion, having travelled a lot, there are many countries where I would happily take a photo of an airport and others were I know I risk jail time. Therefore, in my opinion, it is related to culture, and not to aviation.  Put it this way, to my (uncertain) knowledge there are no regulations in aviation that cover this. All of this said, my opinion doesn't matter. That's why we have a voting system. I have no clue what you imply by "copy/paste".

@Simon You can use a custom flag to suggest that the moderators migrate the question. (But, as I said, I believe this is off-topic on Travel so please don't, in this case.)

I'm voting to close this question as off-topic because it's a question about why some cultures choose to restrict photography of locations that they wish to keep secure (e.g., airports, ports, government buildings). There most likely isn't an objective reason.

@DavidRicherby I disagree.  I can't see why this is off-topic for Travel but I won't use a custom flag.  In my opinion, the lack of a community driven migration vote is a weakness which should not require moderator attention.

@Simon You won't bother moderator to do something for you, which you can't do because there is no proper functionality? Then for what moderators are in your opinion? Sorry, can't see much sense in your statement. I have just kindly asked a moderator to consider suggested migration. I don't think they should be offended with such (t)ask.

@Simon It's been my experience as a regular user of Travel questions along the lines of "Why is X illegal in country Y?" and "Why does company C have policy P?" are held to be off-topic since, in general, there's no single objective reason and, especially in the case of company policies, the actual reasons are usually not known to the public. As for not wanting to flag, flags are precisely for drawing the moderators' attention to things that only they can do.

@DavidRicherby OK, so is there a Stack that would welcome it?

@Simon I'm not aware of any. (Sorry -- would have saved time if I'd said that earlier.)

just a comment from a prior military guy who was briefly assigned to a security unit (but far from an expert): google maps does not have "the same stuff in better detail" than the photo you would take. Gmaps is dated, and does not show what equipment is currently where, which is why photo-recon is used just prior to an attack. Granted, anyone planning an attack is not going to be deterred by signage, and will be more stealthy than just standing at the fence line... but that is a big reason for prohibiting photography.

@FreeMan this question is specific to Abu Dhabi, the FAA and DHS have no authority there.

Google didn't ask permission when they used satellite and overflight data. They will blur on request however.  The same thing for non-public streets driven by streetview drivers.

@Jimmy Your comment sounds like a good explanation and answer.

@FreeMan That's... AUH, not DUH.

I believe the sign is to prevent plane watchers from gathering at the spot and taking pictures of every aircraft taking off or landing, which, judging by the existence of the sign, is not how the location is intended to be used.

It's much more than that.  In most of the Middle East, it is illegal to photograph any area, building or facility considered of military importance or having a high security risk.  Those no photography signs are enforceable.

@Simon not just in the Middle East, in most of the world. This went so far in the USSR we were told to not take photos inside the cabin of civilian airliners because we might inadvertently capture a glimpse of some equipment or (through the window) a postage stamp sized image of a military base 10km below us. At the time, even photographing road bridges and policeman directing traffic at intersections was illegal there.

If you go to plane watching sites there's a lot of discussion about US airports that have problems with photography. Although there are no laws about it that I know of, airports are generally private property and airport security often has a pickle in their butts about people standing at the fence with a camera.

My question and this answer brought so much attention and so many valuable, pure-aviation comments, that I still don't get it, how or why this question is off-topic here.

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/3.0/


text:
What is the air crash featured in Queen's Innuendo official video?
There's a short cameo of a total destruction of a plane in Queen's Innuendo official video (2:11).
Here's the video itself:

And here is a screenshot of the most important frame:

Is anyone able to identify this particular air crash and the airplane involved?

I think a "more important frame" is a few seconds earlier, showing that this is a test aircraft in a controlled crash.

The problem is that only from this point of Queen's video we have a full screen view of the crash. While the moment described by you is shown only on a small screen, in a so called cinema with pseudo-audience.

It was a test crash of a DC-7 conducted by the Flight Safety Foundation under contract to the FAA and in conjunction with NASA (among other organizations) in 1964.
Here is the FAA film, and a NASA report.